1. Field of the Invention
The present invention relates to optical glasses such as waveguide glasses.
2. Discussion of the Related Art
Light propagation losses are a major concern in the art of making optical waveguides and other optical devices. When propagating light through optical glass layers over relatively large distances across substrate surfaces, light is generally lost through scattering and by absorption. Scattering occurs at defects and interfaces when the dimension of inhomogeneities is on the order of the wavelength of the propagated light. Absorption occurs in otherwise nonabsorbing media through the formation of non-stoichiometric compounds during the layer forming process.
It is desirable to reduce light propagation losses in optical glass layers such as waveguides for several reasons. With lower scattering and absorption losses there is a lower requirement for source power. Further, because light propagation losses degrade the dynamic range in integrated optical signal processing devices, improved performance would be achieved in these devices if losses were reduced. Such improved devices would then be more competitive than they now are with devices that use other methods of signal processing.
Optical materials that are used for making glass waveguides, for example, should attentuate light less than 1 dB/cm when prepared as a waveguide layer. Unfortunately, due to difficulties encountered in processing these materials into waveguide layers, this low level of attenuation is not often achieved.
There are known methods of reducing light propagation loss in glass waveguides. For example, S. Dutta, H. E. Jackson and J. T. Boyd, Opt. Eng. 22(1), 117 (1983) report considerable success in using the technique of laser annealing to reduce scattering in a variety of thin-film optical waveguides deposited onto thermally oxidized silicon substrates. Dutta et al. used Corning 7059, a baria borosilicate type glass that is much used in the art.
Laser annealing induces local heating of a selected target area on the glass surface, in contrast to furnace annealing in which the entire glass and its substrate are uniformly heated. The spot size of the localized areas heated in laser annealing depends on the power of the laser and the wavelength of the light, but is generally about 0.01 cm.sup.2 or less. Local heating has the advantage that the glass can reach temperatures close to its melting point while the temperature of the substrate remains much lower. Further, once the source of the heat is removed, cooling of the glass is more rapid in laser annealing than in furnace annealing.
The problem is that while laser annealing offers advantages by annealing only localized areas, it is not a practical method for treating large areas of glass (typically greater than 0.01 cm.sup.2 to about 324 cm.sup.2) as is sometimes necessary for the production processing of devices. Such devices include integrated optical read/write heads, beam deflectors, interferometers such as the Mach-Zender interferometer, waveguide modulators, page scanners, and radio frequency spectrum scanners.